[HLSL] Investigate whether we can avoid memset and memcpy in object construction and type casting

[bogner]

Clang's codegen for C++ constructors doesn't seem to check the target for whether we should generate memcpy for members, and there are also memcpys generated for certain types of cast. We should investigate whether this can and should be made target specific (Especially after #138787), and if so we can eliminate DXIL's custom legalization of memset and avoid implementing memcpy legalization.